Spread spectrum communications systems have been used in a variety of fields. In a communications system of this type, the transmitted bandwidth is much greater than the bandwidth or bit-rate of the information to be transmitted. Additionally, the carrier signal is modulated by some other function than the information being sent. At its essence, spread spectrum communications represents the art of expanding the bandwidth of a signal, transforming that expanded signal, and recovering the desired signal by remapping the received spread spectrum into the original information bandwidth. In general, the purpose of pursuing such a process of spreading information across a wide bandwidth, and the de-spreading of that information at the receiver, is to allow the system to deliver error-free information in a noisy signal environment.
Spread spectrum communications systems have many useful advantages: a selective call is possible since the power spectrum density is low, private communication is allowed, and they are relatively little influenced by interference either due to multipath fading or jamming. Based on these characteristics, spread spectrum systems have found many uses, such as mobile communications systems, avionics systems and satellite communications.
Spread spectrum communications systems can generally be categorized according to one of three types of modulation methods:                1) Modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Such systems are called “direct sequence” modulated systems.        2) Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. Such systems are called “frequency hoppers”.        3) Pulsed-FM or “chirp” modulation in which a carrier is swept over a wide band during a given pulse interval.Of the three modulation techniques discussed, pulsed-FM is used primarily in a radar applications, while direct sequence modulation and frequency hopping are primarily used in communications systems.        
Direct sequence modulation is the simpler of these to implement, with the basic operation being the modulation of the carrier by a code sequence—e.g., PN (pseudo-noise), M-sequence, Gold code and the like—where the code operates to spread the transmitted information across the bandwidth of the system. In a frequency hopping system, on the other hand, a carrier frequency is shifted, or jumped, in discrete increments, in a pattern dictated by a such a code-sequence, in synchronism with a change in the state of the codes. The resulting consecutive and time-sequential frequency pattern is called a hopping pattern.
In reproducing a spread-spectrum information signal at the receiver, a synchronization acquisition process is first performed, in which the code pattern provided in the receiver is made accurately coincident with the code pattern generated in the transmitter, in time position. Then, the spread spectrum signal is de-spread, and thereafter a well known demodulation is performed to extract the desired information. Such spread spectrum transmitting and receiving systems are described in detail in a text entitled Spread Spectrum Systems, by R. C. Dixon, second edition, 1984, published by John Wiley and Sons, Inc.
In Low Probability of Intercept (LPI) communications, which is of particular interest in the field of covert mobile radio communications, a primary object is the security of the communication's link—i.e., avoidance of detection of such communications by third parties which could, in turn, lead either to interception of the transmitted intelligence or interruption of the transmission path by jamming. Both direct sequence modulation and frequency hopping present unique, and largely conflicting, advantages and disadvantages for use in LPI communications.
With direct sequence modulation, because the transmitted energy is spread evenly over the entire bandwidth of a carrier frequency, the energy transmitted at any one frequency within that bandwidth is quite low and relatively unlikely to be detected. However, another characteristic of this modulation process is that of producing a constant amplitude across the entire bandwidth of the carrier signal, and therefore such a signal does not really mimic the noise-like (i.e., random amplitude) signal of low-level noise. Thus, while the direct sequence signal is relatively less likely to be detected, upon detection of such a signal, this constant amplitude characteristic generally presents a clear distinction from a “noise” signal. In addition, spreading of the transmitted information across the entire spectrum of the wide-band carrier creates a disadvantage where other “friendly” transmitters utilizing the same, or similar bandwidths are operating nearby—or, worse, operating at a distance from the intended receiver which is materially less than the distance between that receiver and the sending transmitter. In that circumstance, such an interfering transmitter disrupts the desired communications between the sending transmitter and the intended receiver.
A frequency hopping system, on the other hand, largely avoids the problem of interference from competing transmitters, because the probability of a particular instantaneous transmission frequency in the carrier signal spectrum being used simultaneously by two “competing” transmitters—where selection of such frequencies is established in the usual circumstance by a pseudo-random code—is quite low. The probability of interception, however, for a frequency hopping system is substantially greater than for a direct sequence system because all of the transmitted energy is concentrated in a single frequency at each instant in time. Thus, while the probability of interference from competing “friendly” transmissions is substantially reduced in a frequency hopping system, the likelihood of interception of that transmission by “unfriendly” third parties—and thus possibly decoding the intelligence transmitted and/or jamming the signal—is significantly greater.
At this stage of prior art development, an LPI communications system, where security was a paramount concern, would ordinarily have utilized a direct sequence system. However, as pointed out above, even with a direct sequence system, the constant amplitude waveform across the full spectrum of the carrier signal presented a material likelihood that the signal would be detected. In a 1991 paper entitled “Near Featureless Waveform Spread Spectrum LPI Communications”, the authors, B. Storm and G. Zuelsdorf, describe an approach for a direct sequence system which achieves a noise-like waveform and therefore would be expected to significantly reduce the likelihood of detection for a direct sequence signal. Applicant notes that the authors are employees of the National Security Agency (or at least were at the time the paper was prepared) and that the paper has been classified “secret” by the agency. Applicant does not have sufficient information to determine whether the brief discussion of the subject matter of that paper presented herein as background for Applicant's invention constitutes classified matter.
The essential characteristic of the Storm and Zuelsdorf approach which is needed as background for Applicant's invention is their representation of the coded information as a frequency function which is transformed into the time domain prior to transmission of such information. More specifically, Storm and Zuelsdorf describe a system whereby the frequency band is divided into a large number of frequency channels and a tone is generated in each channel. By varying the phases of such tones, using a pseudo random code, and thereafter transforming those tones to the time domain, the resultant carrier signal is a noise-like waveform—i.e. its amplitude across the frequency spectrum is randomized. Implementation of such a system is accomplished by converting an input signal to the frequency domain using Fourier transforms, randomizing the phases of the resultant tones in accordance with the coding chosen, and then converting such phase-randomized tones back to the time domain for transmission. The receiver, of course, does the reverse operation.
While the Storm and Zuelsdorf approach produces a largely randomized amplitude for the carrier signal of a direct sequence system and should therefore materially reduce the detectability of such a spread spectrum communications signal, the interference problem associated with all direct sequence systems is not addressed—i.e., where an interfering transmitter near to a receiver which is listening to a desired transmitter relatively far away disrupts the desired communication.